1. Field of the Invention
The present invention relates, in general, to lighting units for visual display signs, and, more particularly, to a reflector for a lighting unit using incandescent bulbs.
2. Statement of the Problem
People have long used outdoor signs for advertising, sending messages to others, and displaying artwork. In early times, carved and painted wooden signs were hung outside of taverns and inns to advertise their presence and accessibility for travelers. In more recent times, large billboards are erected near highways and messages are painted on the sides of barns or other large buildings to capture the attention of passing motorists. Such large billboards present an advertisement or message in a static form. That is, the billboard is most often painted or has a previously prepared advertisement attached to it. The message remains on the sign until it is desired to replace the message with a different communication. These billboards are often illuminated during the hours of darkness in order to reach even more viewers. Such illumination is usually achieved with incandescent or other very bright lights.
Many advertisers, however, desire to change their messages more often than is possible with a billboard that must be repainted each time the message is changed. Advertisers also desire to prepare their own unique messages or to use a moving display as opposed to a static picture. Thus, a need exists for large visual display signs showing moving images that can be located outdoors at remote locations such as on a building or a freestanding pylon.
Although visual display systems exist providing moving images in color, for example, a television show or videotape, such display signs have not provided vivid video images, especially those that are used outdoors. Previous signs designed for outdoor use have been prohibitively expensive to operate, to purchase, and to maintain. Lack of brightness is a serious problem for prior outdoor display signs. Light from other sources, especially the sun and the moon, but also including, but not limited to, other lights on the ground or on adjacent buildings, reduces the ability of viewers to perceive the picture displayed by the sign. These outdoor display signs are comprised of a plurality of lighting units, some of which are turned off, and therefore black, at any particular moment, and some of which are lit in order to show the picture. Ambient light reduces the contrast of the sign in two ways. First, when a lighting unit is off, the blackness of the lighting unit is only as dark as the ambient light. Second, when a lighting unit is lit, its colors mix with the ambient light. Thus, external light washes out the picture, causing the viewer to have difficulty perceiving the picture being displayed, especially when that picture is a moving picture in color. Thus, a need exists for a lighting unit that will increase the light output of a visual display sign, without using expensive components. A need exists to reduce operating costs with reduced power requirements and a need exists to reduce maintenance costs.
One method to increase light output from lighting units in visual display signs is to provide the lighting unit with a reflector that reflects the light emitted by the incandescent light source. Many reflectors for such lighting units are parabolic. These parabolic reflectors would provide the most efficient light output from a point source of light. However, the available incandescent light sources are not truly point sources; in currently available light bulbs, the filaments that emit the light comprise a variety of shapes. Thus, a need exists for a lighting unit that has a reflector with a predetermined shape designed to provide maximum light output from a non-point source of incandescent light.
A patentability search for lighting units having a predetermined reflector shape resulted in the following patent. U.S. Pat. No. 2,313,192 to Decalion teaches a reflector with which the maximum flux is utilized for a given depth of the reflector. The reflector is constructed as follows. The light source is positioned on the axis of the reflector at a certain point. If d.sub.1 and d.sub.2 are the distances from that point to the edges of the reflector in a plane passing through the axis, and R.sub.1 and R.sub.2 are the distances of these edges to the axis, the following condition must be met: EQU (d.sub.1 /d.sub.2).sup.3 =(R.sub.1 /R.sub.2).sup.2
In addition to their difficulties in seeing moving pictures on outdoor display signs under lighting conditions that wash out the light emitted from the sign, viewers on the ground often find it difficult to see the picture on a visual display sign when they are viewing the sign from an angle other than directly in front of the sign, for instance, if they are standing below and close to the sign or passing by the side of the sign, because most of the light emitted from the lighting units is directed horizontally from the front of the sign or is diffused too far around the edges of the lighting units. Thus, a need exists for a lighting unit that directs the emitted light downward toward viewers below and in front of the visual display sign and in which the reflector is designed to emit light at a predetermined angle. A patentability search for angled lighting units and for reflectors emitting light at a predetermined angle resulted in the following patents:
U.S. Pat. No. 5,400,228 to Kao discloses a full-color illuminating unit including a circuit board and a plurality of red and green lights disposed around a central blue light. A protrusion above the illuminating unit acts as a shade to block sunlight. The entire illuminating unit is positioned at a downward angle against a vertical backing so that the light emitted from the illuminating unit can project to the ground in an appropriate angle (see FIG. 3).
U.S. Pat. No. 5,410,328 to Yoksza et al. teaches a display comprising a plurality of LED modules. Each module is attached to the vertical wall of the display is such a manner that the module angles downward (see FIG. 2A).
U.S. Pat. No. 4,504,894 to Reibling discloses a reflective surface contoured to control the angle of emanation of the light rays from the lighting unit, with a cutoff of 65 degrees from the vertical axis.
A need therefore exists for an affordable lighting unit for use in visual display signs that provide highly visible moving pictures at the NTSC standard 30 images per second both indoors and outdoors under all lighting conditions. Such a lighting unit should provide a highly efficient light output that is directed at an angle to the ground, so that the pictures displayed can be easily seen by viewers below the sign.
Solution to the Problem
Under the teachings of the present invention, a visual display sign is disclosed having a lighting unit that provides an efficient light output directed toward ground-based viewers. The reflector in this lighting unit is nearly completely sealed, and has a shaped reflecting surface obtained from a set of predetermined curve reference points. The shaped reflecting surface depends on the shape of the filament in the light bulb within the reflector and the desired beam divergence. The reflector having this shaped reflecting surface emits light at an efficiency of about sixty percent (60%). The reflector produces a slightly elliptical spot having eight degrees vertical divergence and twelve degrees horizontal divergence.
The lighting units have a downward angle of about eight degrees from the horizontal. Visual display signs using lighting units according to the present invention provide a bright and consistent image to viewers below or at a distance from the visual display sign.